Workers are aware that many new and innovative high speed line-scanning video imaging systems have recently been developed. Examples are described n U.S. Pat. Nos. 5,063,599, 5,063,461 and 5,144,457. Such systems have typically employed high-precision solid-state imager means (e.g. Charge-Coupled Photo Devices, or CCPDs) to register an optical image and convert it to a digital signal. The development of such CCPD devices has only reached a certain point, and devices of this type which are commercially available have a relatively small sensing area, typically of the order of 1.00" (15 mm) long. Our applications for these devices typically involve the imaging of financial documents, eg bank checks, which are much larger, eg of the order of 5.00" (125 mm) high.
Thus, we require an optical system which reduces the size of the image of the document to fit upon the sensing area of the CCPD. This, in turn, requires that we be able to adjust the position of the CCPD relative to the optical system in several axes, in order to compensate for variations in the CCPD relative to the optical system in several axes, in order to compensate for variations in the CCPD, the optical system and the mechanical parts of the system, and then secure the CCPD in the correct position to a very high degree of accuracy.
Previous designs of high-speed line-scanning video cameras have been intended for use in large, high volume, high cost systems, with generally low rates of production, where the cost of obtaining and securing such adjustments to the CCPD has been less significant when considered as part of the overall cost of the system. Now, however, we are developing imaging systems of far lower cost and also of significantly larger production volumes, and the cost (in both time and money) of obtaining and securing such adjustments has become a critical factor in the overall system development.
Such constraints become difficult to meet when one attempts to apply them to a low-cost imaging system--and this is our problem.
Accordingly, we have developed a new method of so adjusting and fixing the CCPD device as to satisfy the optical and mechanical requirements of a high-speed line-scanning video image camera, while allowing for the use of the latest high-volume, low-cost electronic equipment and production methods.
The CCPD devices used in these cameras are typically low precision in their mechanical assembly. They are typically offered in mechanical packages very similar to the familiar "integrated circuit" or "chip" package, and the location of the critical photosensitive surface inside the package is very poorly related to any external feature of the package. With normal mechanical tolerances, positioning errors of .+-.0.040-0.080" (.+-.1.00-2.00 mm) may easily be generated, with unacceptable effects upon the focus, magnification and alignment of the optical system. Additionally, angular errors of position may also be generated by an accumulation of tolerances, and these are similarly unacceptable to the optical system. Accordingly, some better means of adjusting the CCPD, both in its location and angular positions, must be incorporated in order to suppress these unwanted effects. This is our object here.
We found that the positional location of the CCPD should typically be better than .+-.7 micrometers (.+-.0.000275"), and the angular location of the CCPD should be better than .+-.0.75.degree., in order to reliably produce a system which meets our desired focus, magnification and alignment requirements (i.e. in an optical system which may be reasonably constructed using conventional tolerances, techniques and readily available commercial parts of reasonable cost.) Angular precision is important chiefly in the "roll" and "pitch" directions--because the CCPD device has a sensitive area which has essentially zero width and a wide angle of acceptance; "yaw" variations have no significant impact upon the camera system so long as they are less than approximately .+-.5.0.degree.. Each of these adjustments must be separate and independent of the others, in order to compensate for all potential errors in the positioning of the CCPD.
In previous designs, these adjustments have been obtained by mounting the CCPD on a special, high-precision carrier plate, typically by bonding it thereto with an epoxy adhesive while it is secured in a special adjusting fixture. Such fixtures have typically embodied many precision micrometer-type adjustments for the CCPD, with microscope eyepieces to accurately check the position of the device. Their use demands skilled operators and careful control and calibration of the fixtures, in order to produce a reliable assembly. Additionally, the process is time consuming, both in the adjustment itself, and also in the typically long cure times (several hours) for the epoxy adhesives typically used.
While such techniques are acceptable for large and expensive machines where the rate of production is low, they cannot be used for smaller, lower-cost systems with far higher rates of production. In addition, such techniques give considerable problems in other fields, such as the making of reliable electronic connections to the CCPD device, the high cost of the device as a service part (due to all the hardware, time and labor now associated with it), and the difficulty of ensuring the quality of such an assembly.
More particularly, this innovation involves a technique of mounting a CCPD directly on an associated PCB (printed circuit board), adjusting it in five (5) axes ("X, Y, Z, roll, pitch") and then securing the adjustment using soft-solder and other simple techniques (e.g. as opposed to more conventional epoxy bonding).
Mounting such precision CCPDs presents a challenge to the designer of a camera system. For instance, the:
High precision usually required to give correct mounting position and angle; as noted above. PA1 i) The use of ancillary components (e.g. carrier and adhesive system) between the CCPD and the (camera body) mount surface. To maintain required mounting accuracy, each of these components must be made to very high accuracy, so that their combined tolerances will not exceed the overall acceptable mounting tolerance for the CCPD. The adhesive system should bona the mirror to the carrier using a locating fixture with very high accuracy and corresponding high cost. PA1 ii) The adhesive system must be designed so that it can accommodate the large differential thermal expansion/ contraction between the CCPD and carrier yet such an adhesive may not be sufficiently stiff to maintain the required positional accuracy for the CCPD. PA1 iii) The cost and time (manufacturing) with such a system. Bonding CCPDs to these accuracies requires skilled staff and costly fixtures, and the bonding process may be very time-consuming and require special facilities, such as curing ovens and mixing, dispensing and safety equipment. Additionally, any bonding system is prone to high losses due to contamination of the CCPDs or carriers with adhesive, due to bonding problems, due to poor adhesive preparation, and other like factors which are difficult to control.
Various mounting methods have been contemplated to overcome these problems. One solution might be to mount the CCPD in a special high-precision carrier, typically bonding it in place using a special adjusting fixture, as noted above. While such a system has merit, it has several drawbacks, among which are:
Accordingly, it is an object hereof to ameliorate (at least some of) the foregoing difficulties and provide related advantages, as will become more evident upon considering the following disclosure, in conjunction with the accompanying drawings.